the compound you mention is simple tar. - its useless in a car, btw your car interior will stink like a road for the next 10 or 20 years because you have applied this coating inside.
3M rubberized coating is available in Pakistan which is applied to the underside of the car and is sold exlusively for this purpose, the interior side is finished first with weld through primer and a finish paint then a coat of sound deadening.
What your "denter" is doing is raping the car to extreme.
the piece he "cut" out of the front to replace is wrong practice, the top part where he welded only has 1 inch or less of metal that is spot welded to the fender well - no strength in this work. The correct way was to make a new shell structure and graft the dowel spacers in there (the dowel spacers provide the strength to torque down the front subframe to the car).